you can refer to www.badcaps.net for more info on what to look for. bulging, leaking, domed caps.. Depending on make and model of PSU, that could probably use a look as well.

As far as the BIOS battery, if it's more then a couple years old it's a good idea to go ahead and replace it. If you have a multimeter, you could check the voltage, but I am not certain on what the zone is for what is good.

If the BIOS program was failing I would expect some other form of issue.. like a pixilated boot screen. Not to say that is not the problem, been doing more code stuff versus hardware. If the chip was failing I would expect, similar problems.

Test the BIOS battery in circuit. First, remove the AC mains cord. Attempt a few starts to drain the PSU reserve. Then with a digital multimeter, the top of the cell is marked positive (+) and any bare chassis point or black lead terminal is negative (-). A good battery should show 3 volts. Any lower than 2.8 volts is critical and may cause BIOS settings to default.
That "checksum error" message covers up to 20 critical features.
The BIOS battery is only a redundant feature to retain BIOS settings in the CMOS memory when no power enters the PSU. Even if the PSU is not active, the +5 volt stand-by voltage powers the BIOS and USB circuits.
Depending on draw, the battery may last two years average.

When you get that 'checksum' error message, go into the BIOS to check the date and time plus other settings. If the date and time wander off greatly, suspect a circuit fault that does not provide proper voltage to the BIOS circuit. Measure the +5 volt stand-by voltage at the main power socket.. the violet wire.. to ground. Should be right at +5 volts or maybe +5.3 volts max. A one-way diode from that feed powers the BIOS circuit. Another one-way diode from the battery is a backup. Gold contacts don't corrode or oxidize.. but almost every other metal will. Possible also there is a cold-solder connection that has now reared its menacing head, after many months or years of use. Any solder joint that appears "dull" at the surface can be suspect. But use heat sinks with any sensitive components, if attempting to touch-up any suspect solder joints!
But as evasive noted, inspect all large capacitors for bulging tops, excess leaning, or signs of leakage. The PSU also should be examined!